In gas turbine engines, such as aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel in a combustor. The mixture is then burned and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas expands through the turbine, which in turn spins the shaft and provides power to the compressor. The hot exhaust gases are further expanded through nozzles at the back of the engine, generating powerful thrust, which drives the aircraft forward.
In recent years composite materials have become increasingly popular for use in a variety of aerospace applications because of their durability and relative lightweight. Because engines operate in a variety of conditions, engine components often come into contact with foreign objects, such as hailstones, ice, sand, and dirt. Over time, contact with such foreign objects can damage and erode the surface of the composite engine components.
To help protect the composite components from exposure to such harsh environments, the surface of the component is often coated with external, secondarily applied materials. By “secondarily applied” it is meant that the composite component is cured prior to the application of the coating. These secondarily applied materials may include polymer coatings that are sprayed, painted, or otherwise bonded to the composite component. Other secondarily applied materials may include metal foils or sheets that are preformed and bonded to the composite component. In general, this approach can require considerable labor and expense for preparation and processing, typically requiring sanding and/or priming of the cured composite component prior to the application of the coating.
In addition, to ensure that the finished component satisfies dimensional constraints, each component must be inspected after the protective coating is applied. For example, non-destructive evaluation techniques, such as the use of a coordinate-measuring machine, a hand gauge, or ultrasonic inspection, can be used to determine if defects are present. If it is determined that there is significant coating thickness variation, which there often is, the coating must be stripped and reapplied. Even if the dimensions are found to be accurate, additional labor is often needed to ensure the desired bond integrity and surface finish are achieved.
When it is discovered that a coated surface needs to be repaired, the depth of the eroded portion can be measured to determine the extent of the wear. If the depth of erosion is small, the surface may be locally sanded, prepped, and recoated by spraying or painting a new coating onto the eroded surface. If the depth of erosion is large, entire sections of the composite component may be removed and replaced with patches of material attached using epoxy or other suitable matrix material.
Accordingly, there remains a need for methods allowing for visual inspection of coated components for erosion damage.